1. Field of the Invention
The present invention generally relates to a broadcast method in a wireless network and a communication apparatus using the broadcast method. More particularly, the present invention relates to a broadcast method in a wireless network and a communication apparatus using the broadcast method, which are capable of efficiently broadcasting data by a link-based broadcast in a ZigBee network compliant to the IEEE 802.15.4 standard.
2. Description of the Related Art
In contrast to a Local Area Network (LAN) or a Wide Area Network (WAN), a Personal Area Network (PAN) is a network owned by an individual. Devices owned by the individual are interconnected to construct a network in order to provide convenience for the owner. The Institute of Electrical and Electronics Engineers, Inc. (IEEE) 802.15 Working Group developed the WPAN for short distance wireless networks to standardize and implement the PAN. The IEEE 802.15 standard has four Task Groups. More particularly, IEEE 802.15.1 standardizes the well-known Bluetooth technology, whereas IEEE 802.15.3 and IEEE 802.15.3a standardizes the high rate WPAN. Additionally, IEEE 802.15.4, alias ZigBee, standardizes low rate WPAN which corresponds to data rates less than 250 kbps.
One of the goals of ZigBee is to standardize the protocol stacks above a media access control (MAC) layer for wireless sensor networks. To this end, the current ZigBee specification supports reliable data broadcast at a network layer. A broadcast method specified in the ZigBee specification consists of three major parts, which include a network-wide broadcast, a local broadcast repair, and a limited radius broadcast.
FIGS. 1 through 4 illustrate conventional broadcast methods according to the ZigBee specification. Referring now to FIGS. 1 through 4, the conventional broadcast methods of the ZigBee specification and drawbacks associated with the conventional broadcast methods are described below.
FIG. 1 illustrates the network-wide broadcast mechanism in the ZigBee network which is specified in the ZigBee specification. According to the ZigBee specification, the network-wide broadcast mechanism is a tree-based broadcast in which broadcast packets are only forwarded via a tree structure.
For example, if the source of a broadcast packet is a node A in a network as shown in FIG. 1, the broadcast packet must be transmitted through its parent node in order to reach neighbor nodes because the broadcast packet is delivered only following the tree structure. Thus, transmission of the broadcast packet from node A to node B or node C, as shown in FIG. 1, requires six transmissions and relays following the tree structure. However, by utilizing physical links instead of the tree structure, the broadcast packet can be delivered to node B or node C (of FIG. 1) via three or four nodes along links indicated as dotted lines in FIG. 1. Therefore, the transmission of the broadcast packets following the tree structure according to the ZigBee specification delays the delivery of the broadcast packets unnecessarily.
FIG. 2 illustrates the local broadcast repair mechanism in the ZigBee network. As explained above, the broadcast method of the ZigBee specification utilizes a parent and child relationship based on the three structure. If a node loses contact with its parent node, a broadcast packet is not delivered farther. The local broadcast repair mechanism is used to repair the lost link in such a situation.
The ZigBee specification specifies that a child node, which loses contact with its parent node, performs a local broadcast repair using a ‘Lost Parent’ bit in a packet control field included in a packet header. However, there is no solution with respect to delivery of a broadcast packet from a parent node when the parent node loses contact with the child node. For example, if a link between node C and node B is broken, as shown in FIG. 2, node F can receive a packet from a node, such as node G, via another branch of node C. However, since the packet received from node G is not delivered from its parent or child node, the packet is discarded. As a result, the sub-tree under node B is not covered.
FIG. 3 illustrates the limited radius broadcast mechanism used in a ZigBee network. According to the limited radius broadcast mechanism, a range to which a broadcast packet reaches can be set using a radius counter (RC) value. Referring to FIG. 3, a node 0 performs the limited radius broadcast with RC=2. The RC value decreases by 1 every time the broadcast packet is retransmitted from each node. The broadcast packet is not retransmitted farther when the RC value becomes zero.
The limited radius broadcast, which allows the tree structure, may present undesired results. For example, as shown in FIG. 4, if nodes F and G are one-hop neighbor nodes and node F attempts to perform the limited radius broadcast with RC=3, the broadcast packet will not be delivered to the node G. In order to reach node G, the limited radius broadcast has to be performed with RC=4. That is, the RC value needs to be unnecessarily large to cover neighbor nodes, and therefore, unnecessary traffic increases.